Add access to HttpListnerContext

[gottscj]

In 'HttpRequestEventArgs' the 'HttpListenerContext' is a private field.

Could You add it as a public property?

This will enable me to write a HttpListener based on websocketsharp which is used in a selfhosted OWIN app.
I need to use ASP.NET web api 2, net472 on mono but would like to host websockets on the same port as im hosting my REST api. only think im missing is access to the HttpContext so I can use this when invoking the OWIN pipeline.

I have a branch ready and can create a PR, but I need rigths

thanks

[gottscj]

Added PR #560